Optical transmission device and optical transmission system employing the same

ABSTRACT

An optical transmission device which reduces optical noise in an optical transmission system. The optical transmission device includes a core light amplifying unit, and a first buffer light amplifying unit for amplifying a first signal light from a first transmission path and an amplified second signal light from the core light amplifying unit. The first buffer light amplifying unit supplies the core light amplifying unit with the first signal light, and supplies the first transmission path with the amplified second signal light. Also provided is a second buffer light amplifying unit for amplifying a second signal light from a second transmission path and an amplified first signal light from the core light amplifying unit. The second buffer light amplifying unit supplies the core light amplifying unit with the second signal light, and supplies the second transmission path with the amplified first signal light.

This is a continuation of application Ser. No. 09/663,378, now U.S. Pat.No. 6,314,217 filed Sep. 15, 2000; which is a continuation of Ser. No.09/129,844, filed Aug. 6, 1998, now U.S. Pat. No. 6,195,480.

BACKGROUND OF THE INVENTION

The present invention relates to an optical transmission device and anoptical transmission system. More particularly, the present inventionrelates to an optical transmission device and an optical transmissionsystem suitable for low-noise transmission.

In an attempt to satisfy a requirement of lowering the cost for anoptical transmission system, a wavelength divisional multiplexingoptical transmission system, which transmits different wavelengths ofsignal lights in one single optical transmission fiber, has beenconsidered. In particular, a bi-directional optical transmission system,which transmits different wavelengths of light signals in a singleoptical transmission fiber bi-directionally, is suitable when exchangeof information is needed interactively between the two connectedstations. Under such a technical background, it has become moreimportant to provide an optical amplifier applicable to a bi-directionaloptical transmission system.

Japanese Patent Laid-open No. Hei 6-85369 describes as a conventionalapparatus an optical amplifier. The optical amplifier includes apparatusfor multiplexing or demultiplexing signal lights in a forward or areverse direction toward both ends of a doped fiber. The opticalamplifier is capable of sharing the use of one optical amplifying mediumand one optical pumping source in the forward or the reverse direction,and is applicable to a bi-directional optical transmission system, theconstitution of which is simple.

Japanese Patent Laid-open No. Hei 9-98136 describes another example ofan optical amplifier which is capable of controlling the individualwavelength output even if there occur variations in signal wavelengthmultiplexity.

The optical amplifiers disclosed by the above-identified Japanese patentapplications have various disadvantages in their practical use asdescribed below. It is generally known that, in a one-directionaloptical amplifier having one doped fiber, a signal light input loss at astep previous to the doped fiber is attributed to a degradation in theS/N ratio.

“Optical Amplifiers and Their Application” (Ohm Publishing, May, 1992,pp 5-3[1]), describes that it is essential to combine an opticalisolator at the front of doped fiber for suppressing reflexed amplifiedspontaneous emission (ASE). The optical isolator is not the only opticalcomponent which is inserted at the front of doped fiber. Generally, atransmission equipment requires a wavelength demultiplexer for anoptical surveillance signal, a optical coupler for an optical signalmonitor and a wavelength multiplexer for a pumping light. All of theseoptical components have loses. Further, the noise figure of Erbium dopedfiber having a length of 20-30 m is not negligible. Where the noisefigure is defined by the ratio of the S/N ratio on the input side andthe S/N ratio on the output side.

The optical signal which is attenuated in the transmission path alsosuffers losses due to the optical components. The optical signal isamplified in the EDF of which a noise figure is large. Theabove-described transmission equipment cannot achieve a noise figureless than 6 dB.

When a non-regenerative multiple amplifying transmission is performedusing k units of optical amplifiers, the S/N ratio degradation amountincreases in proportion to the step number k. Accordingly, in an actualoptical transmission system in which there exists an upper limit in thetotal S/N ratio degradation amount, the repeating step number decreasesas the S/N ratio degradation amount in the optical amplifiers increases.This eventually shortens the light transmission distance.

For example, when setting optical amplifiers, the S/N ratio degradationamount of same are 4 dB, and the S/N ratio degradation amount of othersare 6 dB at intervals of 80 km. Under a requirement that the total S/Nratio deterioration amount can not be more than 12 dB, a total S/N ratiodegradation amount of the 4 dB optical amplifiers becomes 12 dB whenthree steps are repeated, and the total S/N ratio degradation amount ofthe 6 dB optical amplifiers becomes 12 dB when two steps are repeated.Thus, when the 4 dB optical amplifiers are used in three repeated stepsit is possible, thus making it possible to transmit a signal light for240 km. Whereas, when the 6 dB optical amplifiers are used in tworepeated steps it is possible to transmit a signal light for 160 km.

SUMMARY OF THE INVENTION

A first object of the present invention is to eliminate theabove-described inconvenience as well as to provide an opticaltransmission device which is applicable to the low-noise opticaltransmission system and is, suppressing a degradation of the S/N ratio,suitable for a long haul optical transmission.

A second object of the present invention is to provide a bi-directionaloptical transmission system suitable for the long distance opticaltransmission.

In order to solve the above-mentioned problems, a terminal stationrepeater or an in-line repeater is configured by at least one bufferlight amplifying unit in contact with a transmission path and at leastone core light amplifying unit in contact with the buffer lightamplifying unit. This configuration allows the buffer light amplifyingunit to amplify an input signal before a signal light, which has beenattenuated because of the propagation along the transmission path,suffers from losses from the optical devices, thereby making it possibleto prevent noise degradation in the optical transmission device.

By use of the present invention it is possible to embody an opticaltransmission device in an optical transmission system, whereindegradation of the S/N ratio is suppressed. Thus, the present inventionis suitable for long haul optical transmission. Further, by employingthe optical transmission device of the present invention it is possibleto develop an optical transmission system suitable for the long distanceoptical transmission.

The present invention provides an optical transmission device whichreduces optical noise in bi-directional transmission systems. Theoptical transmission device includes a core light amplifying unit and afirst buffer light amplifying unit for amplifying a first signal lightfrom a first transmission path and an amplified second signal light fromthe core light amplifying unit. The first buffer light amplifying unitsupplies the core light amplifying unit with the first signal light, andsupplies the first transmission path with the amplified second signallight. A second buffer light amplifying unit is provided for amplifyinga second signal light from a second transmission path and an amplifiedfirst signal light from the core light amplifying unit. The secondbuffer light amplifying unit supplies the core light amplifying unitwith the second signal light, and supplies the second transmission pathwith the amplified first signal light.

The core light amplifying unit includes a first opticalmultiplexer/demultiplexer, a second optical multiplexer/demultiplexer, afirst optical amplifier for amplifying the first signal light from thefirst optical multiplexer/demultiplexer so as to send out the amplifiedfirst signal light to the second optical multiplexer/demultiplexer, anda second optical amplifier for amplifying the second signal light fromthe second optical multiplexer/demultiplexer so as to send out theamplified second signal light to the first opticalmultiplexer/demultiplexer.

BRIEF DESCRIPTION OF THE DRAWINGS

The scope of the present invention will be apparent from the followingdetailed description, when taken in conjunction with the accompanyingdrawings, and such detailed description and specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only, since various changes and modifications within thespirit and scope of the invention will become apparent to those skilledin the art from this detailed description, in which:

FIG. 1 is a basic function block diagram illustrating an embodiment of abi-directional optical transmission system according to the presentinvention;

FIG. 2 is a block diagram illustrating functions of an embodiment of aterminal station repeater according to the present invention;

FIG. 3 is a block diagram illustrating functions of an embodiment of anintermediate repeater according to the present invention;

FIG. 4 is a block diagram for illustrating a configuration and functionsof an embodiment of a terminal station repeater according to the presentinvention;

FIG. 5 is a block diagram illustrating an embodiment of a buffer lightamplifying unit according to the present invention;

FIG. 6 is a block diagram for illustrating a configuration and functionsof an embodiment of a control unit according to the present invention;

FIG. 7 is a block diagram for illustrating a configuration and functionsof an embodiment of a control unit according to the present invention;

FIG. 8 is a diagram illustrating an experimental result obtained byusing an embodiment of a terminal station repeater according to thepresent invention;

FIG. 9 is a diagram illustrating an experimental result obtained byusing an embodiment of a terminal station repeater according to thepresent invention;

FIG. 10 is a diagram illustrating an experimental result obtained byusing an embodiment of a terminal station repeater according to thepresent invention;

FIG. 11 is a block diagram for illustrating a configuration andfunctions of an embodiment of an intermediate repeater according to thepresent invention;

FIG. 12 is a diagram illustrating another embodiment of an intermediaterepeater according to the present invention;

FIG. 13 is a diagram illustrating another embodiment of an intermediaterepeater according to the present invention; and

FIGS. 14 and 15 are block diagrams illustrating uni-directionaltransmission equipment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various modes for carrying out the present invention will be describedbelow using the figures.

FIG. 1 is a basic function block diagram illustrating an embodiment ofan optical transmission system according to a first mode for carryingout the present invention. The optical transmission system includesoptical transmitting units 2 including a plurality of opticaltransmitters 1, optical receiving units 4 including a plurality ofoptical receivers 3, and terminal station repeaters 5. Also, theterminal station repeaters 5 are connected with intermediate repeaters 6through at least one transmission path 7. This configuration allows asignal light to be transmitted bi-directionally from an opticaltransmitting unit 2 ₁ to an optical receiving unit 4 ₂, or from anoptical transmitting unit 2 ₂ to an optical receiving unit 4 ₁.

A line state of the transmission path 7 is supervised in the following amanner: a supervisory light signal introduced by a supervisory unit 8 isintroduced into a transmission path by a supervisory light signalmultiplexer/demultiplexer 9, and in a next terminal station repeater orintermediate repeater, a supervisory light signal launched out from asupervisory light signal multiplexer/demultiplexer 9 is introduced intoa supervisory unit 8, thereby monitoring the line state of thetransmission path. However, the supervisory units are not essential, andeven if they are removed, there are no adverse influence or effects onthe present invention.

A transmission path 7 ₁ is connected with buffer light amplifying units10 ₁, at least one of which is set within a terminal station repeater 5₁. The buffer light amplifying units 10 ₁ are connected through asupervisory light signal multiplexer/demultiplexer 9 ₁ with core lightamplifying units 11 ₁, at least one of which is set within the terminalstation repeater 5 ₁. Further, the core light amplifying units 11 ₁ areconnected with an optical multiplexer/demultiplexer 12 ₁, and at leastone optical transmitter 1 and optical receiver 3. A terminal stationrepeater 5 ₂ is configured in much the same way.

A transmission path 7 ₁ or 7 ₂ is connected with buffer light amplifyingunits 10 ₂ or 10 ₃, at least one of which is respectively set within anintermediate repeater 6 ₁. The buffer light amplifying units 10 ₂ or 10₃ are connected through a supervisory light signalmultiplexer/demultiplexer 9 ₂ or 9 ₃ with core light amplifying units 11₂, at least one of which is set within the intermediate repeater 6 ₁. Anintermediate repeater 6 ₂ is configured in much the same way.

In the present transmission system, an arbitrary number of intermediaterepeaters 6 may be arranged in series. Additionally, in the presentconfiguration, a so-called bi-directional transmission system isassumed, but the similar configuration is also applicable to a unitdirectional transmission system.

This configuration allows the buffer light amplifying units to amplifyan input signal before a signal light, which has been attenuated becauseof the transmission path loss, suffers from a loss from the opticaldevices. Thus, the configuration makes it possible to prevent a noisefigure degradation in the whole optical transmission device. As aresult, it becomes possible to embody an optical transmission systemsuitable for a long haul optical transmission.

The following description is an embodiment of optical transmissiondevices according to a second mode for carrying out the presentinvention as illustrated in FIG. 2. FIG. 2 specifically illustrates theterminal station repeater 5-1, which is one of the components in thebi-directional optical transmission system illustrated in FIG. 1. InFIG. 2, a signal light from the transmission path 7 ₁ is introduced intothe buffer light amplifying unit 10 ₁. A portion of the introducedsignal light is branched by an optical coupler 13. A branched signallight passes through an optical filter 14, which removes a supervisorysignal light, and is then detected by an optical detector 15. Thedetected input supervisory signal is transferred to an optical amplifier16 within the core light amplifying unit 11 ₁.

A signal light having passed through the optical coupler 13 ismultiplexed by an optical multiplexer 17 together with a pumping lightfrom a pumping light source 18, and is then introduced into a rareearth-doped optical fiber 19. Because the rare earth-doped optical fiber19 is raised up to be in an excited state by the pumping light, thesignal light is amplified. The amplified signal light, passing throughthe supervisory light signal multiplexer/demultiplexer 9 ₁, isintroduced into an optical multiplexer/demultiplexer 20 within the corelight amplifying unit 11 ₁. Then, after the signal light is introducedso as to be further amplified into the optical amplifier 16 by way ofthe optical multiplexer/demultiplexer 20, a portion of the signal lightis branched by an optical coupler 22 through an opticalmultiplexer/demultiplexer 21. A branched signal light is detected by anoptical detector 23, and, as an input supervisory unit signal, istransferred to the optical amplifier 16 within the core light amplifyingunit 11 ₁. A signal light having passed through the optical coupler 22reaches the optical multiplexer/demultiplexer 12 ₁. In the opticalmultiplexer/demultiplexer 12 ₁, the signal light is demultiplexed at apredetermined wavelength, reaching the optical receiving unit 4 (notillustrated).

Incidentally, the optical couplers 13, 22, the optical detectors 23, 15,and the optical filter 14 are not necessarily situated at thesepositions. For example, a plurality of them may be set for everytransmission path at a step next to the optical amplifier 16, or at astep next to the optical multiplexer/demultiplexer 12 ₁.

A signal light in the reverse direction on the side of the opticaltransmitting unit 2 ₁ (not illustrated), after being multiplexed by theoptical multiplexer/demultiplexer 12 ₁ within the terminal stationrepeater 5 ₁, a portion of the signal light is branched by the opticalcoupler 22 within the core light amplifying unit 11 ₁. A branched signallight is detected by an optical detector 24, and, as an inputsupervisory unit signal, is transferred to an optical amplifier 25within the core light amplifying unit 11 ₁.

A signal light having passed through the optical coupler 22 is amplifiedthrough the optical. multiplexer/demultiplexer 21 by the opticalamplifier 25. The amplified signal light arrives at the supervisorylight signal multiplexer/demultiplexer 9 ₁ by way of the opticalmultiplexer 20. Then, after passing through the supervisory light signalmultiplexer/demultiplexer 9 ₁, the signal is introduced, so as to befurther amplified, into the rare earth-doped optical fiber 19 within thebuffer light amplifying unit 10 ₁ in a direction opposite to that of theabove-mentioned signal light. The amplified signal light passes throughthe optical multiplexer 17, and then a portion thereof is branched bythe optical coupler 13. A branched signal light, passing through anoptical filter 26 for removing a supervisory unit signal light, isdetected by an optical detector 27, and, as an input supervisory unitsignal, is transferred to an optical amplifier 25 within the core lightamplifying unit 111. A signal light having passed the optical coupler 13is configured to be conveyed into the transmission path 7 ₁.

Here, the optical coupler 22 and the optical detector 24 are notnecessarily situated at these positions. For example, a plurality ofthem may be set for every transmission path at a step previous to theoptical amplifier 25, or at a step previous to the optical multiplexer12 ₁. Also, the optical coupler 13, the optical filter 26, and theoptical detector 27 are not necessarily situated at these positions. Forexample, they may be set at a step next to the optical amplifier 25, ata step previous to the buffer light amplifying unit 10 ₁, or at a stepprevious to the supervisory light signal multiplexer/demultiplexer 9 ₁.

Meanwhile, a supervisory light signal on the side of a supervisory lightsignal source (not illustrated), which is introduced by the supervisorylight signal multiplexer/demultiplexer 9 ₁ and passing through thebuffer light amplifying unit 10 ₁, is introduced into the transmissionpath. Additionally, the optical amplifiers 16, 25 are configured to becontrolled by the input supervisory unit signal and the outputsupervisory unit signal.

In a terminal station repeater based on the conventional apparatus,there exist optical losses caused by optical devices such as the opticalmultiplexer 20 set at a step previous to the optical amplifier 16, thesupervisory light signal multiplexer/demultiplexer 9 ₁, and opticalisolators set within the optical amplifier 16. The optical lossesresulted in a factor of bringing about a noise figure degradation in thewhole terminal station repeater. In the present invention, however, thebuffer light amplifying unit 10 ₁ is configured to amplify an inputsignal before an attenuated signal light from the transmission path 7 ₁suffers from the losses due to the optical devices. Thus, the presentinvention makes it possible to prevent the noise figure degradation inthe whole terminal station repeater.

At the same time, according to the buffer light amplifying unit 10 ₁ inthe present configuration, it becomes unnecessary to employ the opticalisolators, since the buffer amplifier is of low gain. the opticalisolators were essential to an optical amplifier in the conventionalapparatus. This makes it possible to prevent a noise figure degradationin the buffer light amplifying unit 10 ₁ itself, and eventually makes itpossible to prevent the noise figure degradation in the whole terminalstation repeater 5 ₁.

Described below using FIG. 3 is another embodiment of opticaltransmission devices according to a second mode for carrying out thepresent invention:

FIG. 3 is a configuration diagram illustrating the intermediate repeater6 ₁, which is one of the components in the bi-directional opticaltransmission system indicated in FIG. 1. A signal light from thetransmission path 7 ₁ is introduced into the buffer light amplifyingunit 10 ₂. A portion of the introduced signal light is branched by anoptical coupler 28. A branched signal light passes through an opticalfilter 29 for removing a supervisory unit signal light, and is thendetected by an optical detector 30. The detected input supervisory unitsignal is transferred to an optical amplifier 31 within a core lightamplifying unit 11 ₂.

A signal light having passed through the optical coupler 28 ismultiplexed by an optical multiplexer 32 together with a pumping lightfrom a pumping light source 33, and is then introduced into a rareearth-doped optical fiber 34. Because the rare earth-doped optical fiber34 is raised up to be in an excited state by the pumping light, thesignal light is amplified. The amplified signal light, passing through asupervisory light signal multiplexer/demultiplexer 9 ₂, is introducedinto an optical multiplexer/demultiplexer 35 within the core lightamplifying unit 11 ₂. Then, after being introduced so as to be furtheramplified into an optical amplifier 36 by the opticalmultiplexer/demultiplexer 35, the signal light passes through asupervisory light signal multiplexer/demultiplexer 9 ₃ by way of anoptical multiplexer/demultiplexer 37 and is introduced into a rareearth-doped optical fiber 38 within a buffer light amplifying unit 103.

The rare earth-doped optical fiber 38, into which an optical multiplexer40 multiplexes and introduces a pumping light from a pumping lightsource 39, lies in an excited state. Consequently, the signal light isamplified and, passing through the optical multiplexer 40, a portionthereof is branched by an optical coupler 41. A branched signal light,passing through an optical filter 42 for removing a supervisory unitsignal light, is detected by an optical detector 43, and, as an outputsupervisory unit signal, is transferred into the optical amplifier 36within the core light amplifying unit 11 ₂. A signal light having passedthrough the optical coupler 41 is conveyed into a transmission path 7 ₂.

However, the optical coupler 41, the optical filter 42, and the opticaldetector 43 are not necessarily situated at these positions. Forexample, they may be set at a step next to the optical amplifier 36, ata step previous to the buffer light amplifying unit 10 ₃, or at a stepprevious to the supervisory light signal multiplexer/demultiplexer 9 ₃.

A signal light in the reverse direction on the side of the transmissionpath 7 ₂ is introduced into a buffer light amplifying unit 10 ₃. Aportion of the introduced signal light is branched by the opticalcoupler 41. A branched signal light, passing through an optical filter44 for removing a supervisory unit signal light, is detected by anoptical detector 45. The detected input supervisory unit signal istransferred into the optical amplifier 31 within the core lightamplifying unit 11 ₂.

A signal light having passed through the optical coupler 41 ismultiplexed by the optical multiplexer 40 together with a pumping lightfrom the pumping light source 39, and is then introduced into a rareearth-doped optical fiber 38. Because the rare earth-doped optical fiber38 is raised up to be in an excited state by the pumping light, thesignal light is amplified. The amplified signal light, passing throughthe supervisory light signal multiplexer/demultiplexer 9 ₃, isintroduced into an optical multiplexer/demultiplexer 37 within the corelight amplifying unit 11 ₂. Then, after being introduced so as to befurther amplified into an optical amplifier 31 by the opticalmultiplexer/demultiplexer 37, the signal light passes through thesupervisory light signal multiplexer/demultiplexer 9 ₂ by way of theoptical multiplexer/demultiplexer 35 and is introduced into the rareearth-doped optical fiber 34 within the buffer light amplifying unit 10₂. The rare earth-doped optical fiber 34, into which an opticalmultiplexer 32 multiplexes and introduces a pumping light from thepumping light source 33, lies in an excited state. Consequently, thesignal light is amplified and, passing through the optical multiplexer32, a portion thereof is branched by the optical coupler 28. A branchedsignal light, passing through an optical filter 46 for removing asupervisory unit signal light, is detected by an optical detector 47,and, as an output supervisory unit signal, is transferred into theoptical amplifier 31 within the core light amplifying unit 11 ₂. Asignal light having passed through the optical coupler 28 is conveyedinto the transmission path 7 ₁.

However, the optical coupler 28, the optical filter 46, and the opticaldetector 47 are not necessarily situated at these positions. Forexample, they may be set at a step next to the optical amplifier 31, ata step previous to the buffer light amplifying unit 10 ₂, or at a stepprevious to the supervisory light signal multiplexer/demultiplexer 9 ₂.

Meanwhile, a supervisory light signal on the side of a supervisory lightsignal source (not illustrated), which is introduced by the supervisorylight signal multiplexers/demultiplexers 9 ₂, 9 ₃ and, passing throughthe buffer light amplifying units 10 ₂, 10 ₃, is introduced into thetransmission paths 7 ₁, 7 ₂. In addition, the optical amplifiers 31, 36are configured to be controlled by the input supervisory unit signal andthe output supervisory unit signal.

In a terminal station repeater based on the conventional apparatus,there exist optical losses caused by optical devices such as the opticalmultiplexers 35, 37 set at a step previous to the optical amplifiers 31,36, the supervisory light signal multiplexers/demultiplexers 9 ₂, 9 ₃,and optical isolators set within the optical amplifiers 31, 36. Theoptical losses resulted in a factor of bringing about a noise figuredegradation in the whole terminal station repeater. In the presentinvention, however, the buffer light amplifying unit 10 ₂, or 10 ₃ isconfigured to amplify an input signal before an attenuated signal lightfrom the transmission paths 7 ₁, 7 ₂ suffers from the losses due to theoptical devices, thus making it possible to prevent the noise figuredegradation in the whole terminal station repeater.

At the same time, according to the buffer light amplifying units 10 ₂,10 ₃ in the present configuration, it becomes unnecessary to employ theoptical isolators, which were essential to an optical amplifier in theprior art. This makes it possible to prevent a noise figure degradationin the buffer light amplifying units 10 ₂, 10 ₃ themselves, too, andeventually makes it possible to prevent the noise figure degradation inthe whole terminal station repeater 6 ₁.

Described below using FIG. 4 is still another embodiment of opticaltransmission devices according to a second mode for carrying out thepresent invention.

Here, FIG. 4 is a block diagram illustrating functions of a terminalstation repeater. FIG. 5 is a block diagram illustrating a buffer lightamplifying unit, and FIG. 6 is a block diagram illustrating functions ofa control unit. Also, FIG. 8, and FIG. 9 or FIG. 10 are diagramsindicating experimental results obtained by using a terminal stationrepeater.

In FIG. 4, the signal light includes the four wavelengths: λ1=1530.33nm, λ2=1531.90 nm, λ3=1533.47 nm, and λ4=1535.04 nm. Moreover, a probelight is transmitted by a probe light source 48 with a wavelength ofλp1=1543.73. Meanwhile, the four wavelengths are received: λ5=1555.75nm, λ6=1557.36 nm, λ7=1558.98 nm, and λ8=1560.61 nm. Furthermore, aprobe light is received by a probe light receiver (not illustrated) witha wavelength of λp2=1546.92.

Each of the wavelengths of λ1 to λ4 is branched by optical couplers 22-1to 22-4 each of the branching ratios of which is 5:95, and isrespectively detected by optical detectors 24 ₁ to 24 ₄. An inputsupervisory unit signal for each of the detected wavelengths istransferred into a control unit 49 described hereinafter inside anoptical amplifier 25. The signal lights and the probe light, which havepassed through optical couplers 22 ₁ to 22 ₄, are multiplexed by anoptical multiplexer 50 inside an optical multiplexer/demultiplexer 12 ₁,and passes through a dispersion compensator 51 inside an opticalamplifier 25. The dispersion compensator 51 compensates dispersioncharacteristics which a signal light causes when passing throughtransmission paths 7 ₁ to 7 ₄. The multiplexed lights having passedthrough the dispersion compensator 51 pass through an optical isolator52, then being introduced into a rare earth-doped optical fiber 53.

The rare earth-doped optical fiber 53 is in an excited state, since apumping light has been introduced therein through an optical multiplexer55 by a pumping light source 54, which is a semiconductor laser havingthe oscillation wavelength in proximity to 1480 nm. Accordingly, themultiplexed lights are amplified, and passing through an opticalisolator 56, an optical multiplexer 20, and the supervisory light signalmultiplexer/demultiplexer 9 ₁, they are introduced into a buffer lightamplifying unit 10 ₁. The supervisory light signalmultiplexer/demultiplexer 9 ₁ multiplexes the supervisory light signalat 1.48 μm wavelength and the signal lights.

The multiplexed lights introduced into the buffer light amplifying unit10 ₁ is introduced into an erbium-doped optical fiber as a rareearth-doped optical fiber 19, into which a pumping light has beenintroduced through an optical multiplexer 17 from a semiconductor laser(a pumping light source 18) having the oscillation wavelength inproximity to 980 nm. Although the erbium-doped optical fiber 19 is beingin an exited state, the lights which can be amplified are themultiplexed lights at λ1 to λ4 wavelengths and the probe light only. Thesupervisory light signal at 1.48 μm passes through the fiber aftersuffering from some loss. Also, the pumping light source 18 is monitoredby an optical detector 57 for detecting a portion of the optical outputfrom the pumping light source 18. At that time, a control unit 58 isconfigured to control the devices so that the pumping light sourcesupervisory unit signal remains unchanged.

The amplified multiplexed lights and the supervisory light signal at1.48 μm are partially branched by an optical coupler 13 the branchingratio of which is 5:95. A portion of the probe light, which has passedthrough a narrow bandwidth optical filter 26 allowing the probe light topass through, is detected by an optical detector 27. The detected probelight supervisory unit signal is conveyed to the above-mentioned controlunit 49. The control unit 49 is configured to control the pumping lightsource 54 so that the probe light supervisory unit signal remainsunchanged. In this way, by controlling the devices so that the probelight supervisory unit signal remains unchanged, it becomes possible tocontrol and maintain all the signal lights at λ1 to λ4 wavelengths atfixed outputs.

Namely, if any of the signal lights at λ1 to λ4 wavelengths shut down,or even if a signal light other than the signal lights at λ1 to λ4wavelengths is newly added, no influences are exerted on optical outputsof the signal lights at λ1 to λ4 wavelengths (for example, when a signallight at λ4 is cut off, signal lights at λ1 to λ3). This always makes itpossible to perform a fixed and stable control of the device.

The amplified multiplexed lights and the supervisory light signal havingpassed through the optical coupler 13 are transferred to thetransmission path 7 ₁, which is a single mode transmission fiber.

Here, the dispersion compensator 51 may be omitted when dispersioncharacteristics of the transmission paths exerts no influence ontransmission characteristics of the whole system. Also, a place at whichthe dispersion compensator 51 is to be set does not necessarily coincidewith this position. A part consisting of the rare earth-doped opticalfiber 53, the pumping light source 54, and the optical multiplexer 55may be replaced by a semiconductor optical amplifier. In this case, itis advisable that an amplification ratio is controlled by a pumpingelectric current instead of the pumping light source 54. As is much thesame way, a part consisting of the rare earth-doped optical fiber 19,the pumping light source 18, and the optical multiplexer 17 may bereplaced by a semiconductor optical amplifier.

On the other hand, reverse-directional signal lights at λ5 to λ8 and areverse-directional multiplexed light at λp2, which are transmitted fromthe transmission fiber 7 ₁, and the supervisory light signal at 1.48 μmare partially branched by the optical coupler 13 the branching ratio ofwhich is 5:95. A portion of the probe light, which has passed through anarrow bandwidth optical filter 14 allowing the probe light to passthrough, is detected by an optical detector 15. The detected inputsupervisory unit signal is conveyed to a control unit 59 describedhereinafter. The multiplexed lights and the supervisory light signalhaving passed through the optical coupler 13 are multiplexed with apumping light from the semiconductor laser as the pumping light source18 having the oscillation wavelength in proximity to 980 nm by theoptical multiplexer 17 inside the buffer light amplifying unit 10 ₁,thus being amplified by the erbium-doped optical fiber 19. In this case,too, the lights which can be amplified are the multiplexed lights at λ5to λ8 and at λp2 only. The supervisory light signal at 1.48 μm passesthrough the fiber after suffering from some loss. The supervisory lightsignal at 1.48 μm having passed through the fiber is demultiplexed bythe supervisory light signal multiplexer/demultiplexer 9 ₁, then beingtransmitted into a supervisory light signal path. The multiplexedlights, having passed through the optical multiplexer 20 and an opticalisolator 60 inside the optical amplifier 16, are multiplexed by anoptical multiplexer 62 with a pumping light from a semiconductor laseras a pumping light source 61 having the oscillation wavelength inproximity to 980 nm, thus being amplified by an erbium-doped opticalfiber 63. Also, the pumping light source 61 is monitored by an opticaldetector 64 for detecting a portion of the optical output from thepumping light source 61. At that time, a control unit 65 is configuredto control the devices so that the pumping light source supervisory unitsignal remains unchanged.

The amplified signal lights, passing through an optical isolator 66, areintroduced into a dispersion compensator 67. After being amplified by asecond erbium-doped optical fiber 68, the signal lights pass through anoptical multiplexer 69, then being outputted from an optical isolator70. The second erbium-doped optical fiber 68 is in an exited state,since it is multiplexed with a pumping light from a semiconductor laser(a second pumping light source 71) having the oscillation wavelength inproximity to 980 nm. The multiplexed lights from the optical isolator 70are partially branched by an optical coupler 72 the branching ratio ofwhich is 5:95. Branched multiplexed lights pass through a narrowbandwidth optical filter 73 allowing the probe light to pass through,and a portion of the probe light is detected by an optical detector 74.The detected output supervisory unit signal is conveyed to the controlunit 59 inside the optical amplifier 16. At that time, a pumping lightsource 71 is configured to be controlled so that the output supervisoryunit signal remains unchanged.

Multiplexed lights having passed through the optical coupler 72 aredemultiplexed for each of the wavelengths of λ5 to λ8 by an opticaldemultiplexer 75. The each wavelength is branched by optical couplers 22₅ to 22 ₈ each of the branching ratios of which is 5:95, and isrespectively detected by optical detectors 23 ₁ to 23 ₄. An outputsupervisory unit signal for each of the detected wavelengths istransferred into the control unit 59 inside the optical amplifier 16. Asignal light at each of the wavelengths having passed through theoptical couplers 22 ₅ to 22 ₈ is conveyed to a terminal station unit(not illustrated).

In the present configuration, a signal input power into the buffer lightamplifying unit 10 ₁ from the transmission path 7 ₁ falls in a range of−30 dBm to −5 dBm, and a signal amplification gain in the buffer lightamplifying unit 10 ₁ is equal to an order of about 10 dB. Since there isfurnished no optical isolator within the buffer light amplifying unit 10₁, attention must be paid to oscillation phenomena of light.Accordingly, the signal amplification gain in the buffer lightamplifying unit 10 ₁ should be, preferably, 30 dB or less, or morepreferably, 15 dB or less. Also, by making a positive gain the signalamplification gain in the buffer light amplifying unit 10 ₁, a noisefigure for a signal input from the transmission path 7 ₁ in the terminalstation repeater is obviously improved as compared with the methods inthe prior art, but more preferably, it should be 5 dB or more.

Moreover, it is preferable that amplification gain distributions in thecore light amplifying unit 11 ₁ and the buffer light amplifying unit 10₁ should be calculated from a necessary output power into thetransmission path 7 ₁. For example, assuming that the output power intothe transmission path is equal to +11 dBm per signal wavelength, thetotal signal power (λ1 to λ4 and λp2) turns out to be +8 dBm, andconsequently it is preferable that a power of the pumping light source18 should be set to be about 1.25 to 3.3 times as high as this power.When the power of the pumping light source is not enough, as illustratedin FIG. 5, the following units may be added, thereby providing abi-directional pumping for the erbium-doped optical fiber: a new pumpinglight source 18-a, an optical detector 57-a for detecting the opticaloutput thereof, a control unit 58-a for keeping a detected supervisoryunit signal unchanged, and an optical multiplexer 17-a for introducingthe pumping light. Besides, in any case, it is preferable that thepumping light source 18, which corresponds to a forward pumping for themultiplexed lights from the transmission path 7 ₁, is furnished.

Based on the ability of the pumping light source 18 set above, it ispossible to set an input power of the multiplexed lights, which areconveyed into the buffer light amplifying unit 10 ₁ from the opticalamplifier 25, at the value of [the optical output from the buffer lightamplifying unit 10-1 (+11 dBm)−X dB]. It is preferable that a range of Xshould be 0 to 20. An adjustment of X makes it possible to set, at theabove-mentioned more preferable value, a signal amplification gain for asignal input power which is reverse-directional, i.e. in a directionfrom the transmission path 7 ₁.

Here, a 980 nm semiconductor laser may be employed as the pumping lightsource 54 inside the optical amplifier 25. Also, a 1480 nm semiconductorlaser may be employed as the pumping light source 71 inside the opticalamplifier 16.

However, an employment of the 980 nm semiconductor laser is best suitedfor the pumping light source 18 inside the buffer light amplifying unit10 ₁ and the pumping light source 61 ₁ inside the optical amplifier 16.

Described below with reference to FIG. 6 is a configuration of anembodiment of the control unit 49.

An output supervisory unit signal transmitted into the control unit 49has been compared with a predetermined reference value 77 by a comparingunit 76. A pumping light source 54 (not illustrated) is controlled bytransmitting an error signal relative to the reference value 77 to adriving circuit 78.

Also, input supervisory unit signals corresponding to λ1 to λ4transmitted into the control unit 49 have been respectively comparedwith a reference value 79 by a comparing unit 80. When they are higherthan the predetermined value, a normal signal is transmitted to awavelength number detection circuit 81, and when they are lower, anabnormal signal is transmitted. The wavelength number detection circuit81 counts the wavelength number of the transmitted normal signal, thusjudging the wavelength number which can be transferred at the moment.When there turns out to be no wavelength which can be transferred, thewavelength number detection circuit issues an alarm. Also, at that time,the alarm is transmitted to the driving circuit 78, too. Having receivedthe alarm, the driving circuit 78 is configured to control and halt thepumping light source 54 (not illustrated).

Another configuration of an embodiment of the control unit 59 will bedescribed hereunder and illustrated in FIG. 7.

An output supervisory unit signal transmitted into the control unit 59has been compared with a predetermined reference value 83 by a comparingunit 82. The pumping light source 71 (not illustrated) is controlled bytransmitting an error signal relative to the reference value 83 to adriving circuit 84.

Output supervisory unit signals corresponding to λ5 to λ8 and an inputsupervisory unit signal corresponding to λp2 transmitted into thecontrol unit 59 have been respectively compared with a reference value85 by a comparing unit 86. When they are higher than the predeterminedvalue, a normal signal is transmitted to a wavelength number detectioncircuit 87, and when they are lower, an abnormal signal is transmitted.The wavelength number detection circuit 87 counts the wavelength numberof the transmitted normal signal, thus judging the wavelength numberwhich can be transferred at the moment. When there turns out to be nowavelength which can be transferred, the wavelength number detectioncircuit issues an alarm. Also, at that time, the alarm is transmitted tothe driving circuit 84, too. Having received the alarm, the drivingcircuit 84 is configured to control and halt the pumping light source 71(not illustrated). The control may be executed so that the alarm isissued even when the signal which can be transferred is the onecorresponding to λp2 only.

A characteristic in the buffer light amplifying unit 10 ₁ according tothe present invention is to introduce multiplexed lights into the rareearth-doped optical fiber 19 from bi-directions and then amplify themultiplexed lights bi-directionally. Also, another characteristic is toamplify attenuated multiplexed lights introduced from the transmissionpath 7 ₁ before they suffer from considerable losses.

As described above, the control unit 58 controls and maintains an outputof the pumping light source 18 at a fixed value. This makes it possibleto allow the buffer light amplifying unit 10 ₁ to function as a lightamplifying unit having an approximately constant gain as well as tomaintain a stable and lowered noise figure of the buffer lightamplifying unit. Controlling an output of the pumping light source 18 ata fixed value is important for reducing a wavelength dependence of thegain, which rare earth-doped optical fibers generally have, and forsuppressing a wavelength deviation variation of the gain, which turnsout to be a problem in the transmission characteristics.

Furthermore, as described above, the control unit 49 controls andmaintains an output of the probe light at a fixed value. This makes itpossible to automatically control outputs of multiplexed lights into thetransmission path 7 ₁ inside the buffer light amplifying unit 10 ₁.

FIGS. 8, 9 and 10 illustrate the results of an experiment in which theinput/output characteristics and a noise figure of the signal light aremeasured in the present embodiment when operated. More particularly,FIGS. 8 and 9 illustrate the results of the experiment on the wholesystem including a buffer light amplifier and a core light amplifier.FIG. 10 illustrates the results of the experiment on only the bufferlight amplifier.

The measurement points of the experiment are explained by using FIG. 4.The input power of FIG. 8 is the signal level from transmission line171. The output power of FIG. 8 is the signal level after anamplification of the first stage of optical amplifier 16. ASE LEVEL isthe value that divided the power of amplified spontaneous emission light(ASE) of the 1550 nm by the wavelength. The ASE level is used for thecalculation of the noise figure. This experiment is implemented usingthe following conditions: (a) the reverse signal of about +11 dBm, thatis amplified by the optical amplifier 25, is introduced to the bufferlight amplifier 10 ₁; and (b) signal output of +17 dBm is alwaysdelivered to the transmission line 17 ₁. Under the above conditions, theoutput power of the pumping light source for the buffer light amplifieris supplied at about 110 mW, that is the equivalent of 2.2 times of +17dBm, which is signal output power.

According to FIG. 9, which illustrates a graph of the gain and noisefigure against the input power, it is clear that noise figure againstthe input signal from the transmission path is controlled to 3.9 dB orless. Even if the temperature fluctuation is a non-experimental systemand dispersion at the time of manufacturing are considered, according tothe present invention, it can be controlled to 4.5 dB or less. Inaddition, it can be made less than 4.0 dB by maintaining experimentalstructure in the actual system.

The excitation power of semiconductor laser 61 ₁ of the pumping lightsource of the first stage of optical amplifier 16 was made 75 mW in thisexperiment. Therefore, output power cannot be constantly controlled.However, it is possible to constantly control output power by making theexcitation power of a semiconductor laser 61 ₁ 100-120 mW.

FIG. 10 illustrates that the input signal gain of the buffer lightamplifier 10 ₁ maintains 10 dB constantly under the condition of signaloutput of reverse direction that is maintained at +17 dBm.

Described below using FIG. 11 is an even further embodiment of opticaltransmission devices according to a second mode for carrying out thepresent invention. FIG. 11 is a block diagram which illustrates aconfiguration and functions of an intermediate repeater.

Signal lights at the four wavelengths are transmitted from atransmission path 7-1: λ1=1530.33 nm, λ2=1531.90 nm, λ3=1533.47 nm, andλ4=1535.04 nm. Moreover, a probe light at λp1=1543.73 is transmitted.Meanwhile, the four wavelengths are transmitted from a transmission path7 ₂: λ5=1555.75 nm, λ6=1557.36 nm, λ7=1558.98 nm, and λ8=1560.61 nm.Furthermore, a probe light at λp2=1546.92 is transmitted.

The present embodiment differs from the terminal station repeaterillustrated in that Buffer light amplifying units 10 ₂, 10 ₃ are set atthe both ends of a core light amplifying unit 11 ₂, and the core lightamplifying unit 11 ₂ is configured by two units of optical amplifiers31, 36, each of which has the same configuration as that of the opticalamplifier 16 in FIG. 4.

The signal lights at λ1 to λ4 and a multiplexed light at λp1, which aretransmitted from the transmission fiber 7 ₁, and a supervisory lightsignal at 1.48 μm are partially branched by an optical coupler 28 thebranching ratio of which is 5:95. A portion of the probe light, whichhas passed through a narrow bandwidth optical filter 29 allowing a probelight to pass through, is detected by an optical detector 30. Thedetected input supervisory unit signal is conveyed to a control unit 59₂ described hereinafter. The multiplexed lights and the supervisorylight signal having passed through the optical coupler 28 aremultiplexed with a pumping light from a semiconductor laser as a pumpinglight source 33 having the oscillation wavelength in proximity to 980 nmby an optical multiplexer 32 inside the buffer light amplifying unit 10₂, thus being amplified by an erbium-doped optical fiber as a rareearth-doped optical fiber 34. At that time, the erbium-doped opticalfiber 34 is in an exited state, but the lights which can be amplifiedare the multiplexed lights at λ1 to λ4 and the probe light only. Thesupervisory light signal at 1.48 μm passes through the fiber aftersuffering from some loss.

Also, the pumping light source 33 is monitored by an optical detector 57₂ for detecting a portion of the optical output from the pumping lightsource 33. At that time, a control unit 58 ₂ is configured to controlthe devices so that the pumping light source supervisory unit signalremains unchanged.

The supervisory light signal at 1.48 μm having passed through the fiberis demultiplexed by a supervisory light signal multiplexer/demultiplexer9 ₂, then being transmitted into a supervisory light signal path. Themultiplexed lights, having passed through an optical multiplexer 35 andan optical isolator 60 ₂ inside the optical amplifier 36, aremultiplexed by an optical multiplexer 62 ₂ with a pumping light from asemiconductor laser as a pumping light source 61-2 having theoscillation wavelength in proximity to 980 nm, thus being amplified byan erbium-doped optical fiber 63 _(2.) Also, the pumping light source 61₂ is monitored by an optical detector 64 ₂ for detecting a portion ofthe optical output from the pumping light source 61 ₂. At that time, acontrol unit 65 ₂ is configured to control the devices so that thepumping light source supervisory unit signal remains unchanged.

The amplified signal lights, passing through an optical isolator 66 ₂,are introduced into a dispersion compensator 67 ₂. After being amplifiedby a second erbium-doped optical fiber 68 ₂, the signal lights passthrough an optical multiplexer 69 ₂, then being outputted from anoptical isolator 70 ₂. Since it is multiplexed with a pumping light froma semiconductor laser as a second pumping light source 71 ₂ having theoscillation wavelength in proximity to 980 nm, the second erbium-dopedoptical fiber 68 ₂ is in an exited state. The multiplexed lights fromthe optical isolator 70 ₂, passing through an optical multiplexer 37 anda supervisory light signal multiplexer/demultiplexer 9 ₃, are introducedinto the buffer light amplifying unit 10 ₃. The supervisory light signalmultiplexer/demultiplexer 9 ₃ multiplexes the supervisory light signalat 1.48 μm wavelength and the signal lights.

The multiplexed lights introduced into the buffer light amplifying unit10 ₃ is introduced into an erbium-doped optical fiber as a rareearth-doped optical fiber 38, into which a pumping light has beenintroduced through an optical multiplexer 40 from a semiconductor laseras a pumping light source 39 having the oscillation wavelength inproximity to 980 nm. Although the erbium-doped optical fiber 38 is beingin an exited state, the lights which can be amplified are themultiplexed lights at λ1 to λ4 wavelengths and the probe light only. Thesupervisory light signal at 1.48 μm passes through the fiber aftersuffering from some loss.

Also, the pumping light source 39 is monitored by an optical detector 57₃ for detecting a portion of the optical output from the pumping lightsource 39. At that time, a control unit 58 ₃ is configured to controlthe devices so that the pumping light source supervisory unit signalremains unchanged. The amplified multiplexed lights and the supervisorylight signal at 1.48 μm are partially branched by an optical coupler 41the branching ratio of which is 5:95. A portion of the probe light,which has passed through a narrow bandwidth optical filter 42 allowing aprobe light to pass through, is detected by an optical detector 43. Thedetected probe light supervisory unit signal is conveyed to theabove-mentioned control unit 59 ₂. The control unit 59 ₂ is configuredto control the pumping light source 71 ₂ so that the probe lightsupervisory unit signal remains unchanged. In this way, by controllingthe devices so that the probe light supervisory unit signal remainsunchanged, it becomes possible to control and maintain all the signallights at λ1 to λ4 wavelengths at fixed outputs.

If any of the signal lights at λ1 to λ4 wavelengths is cut off, or evenif a signal light other than the signal lights at λ1 to λ4 wavelengthsis newly added, no influences are exerted on optical outputs of thesignal lights at λ1 to λ4 wavelengths (for example, when a signal lightat λ4 is cut off, signal lights at λ1 to λ3). This always makes itpossible to perform a fixed and stable control of the device.

The amplified multiplexed lights and the supervisory light signal havingpassed through the optical coupler 41 are transferred to thetransmission path 7-2, which is a single mode transmission fiber.

A part including the rare earth-doped optical fibers 34, 63 ₂, 68 ₂, 38,the pumping light sources 33, 61 ₂, 71 ₂, 39, and the opticalmultiplexers 32, 62 ₂, 40 may be replaced by a semiconductor opticalamplifier. In this case, it is advisable that an amplification ratio iscontrolled by a pumping electric current instead of the pumping lightsources 33, 61 ₂, 71 ₂, 39.

On the other hand, reverse-directional signal lights at λ5 to λ8 and areverse-directional multiplexed light at λ2, which are transmitted fromthe transmission fiber 7 ₂, and a supervisory light signal at 1.48 μmare partially branched by the optical coupler 41 the branching ratio ofwhich is 5:95. The wavelength of the supervisory light signal can alsobe 1.51 μm. A portion of the probe light, which has passed through anarrow bandwidth optical filter 44 allowing a probe light to passthrough, is detected by an optical detector 45. The detected inputsupervisory unit signal is conveyed to a control unit 59 ₃ describedhereinafter.

The multiplexed lights and the supervisory light signal having passedthrough the optical coupler 41 are multiplexed with a pumping light fromthe semiconductor laser as the pumping light source 39 having theoscillation wavelength in proximity to 980 nm by the optical multiplexer40 inside the buffer light amplifying unit 10 ₃, thus being amplified bythe erbium-doped optical fiber as the rare earth-doped optical fiber 38.At that time, the erbium-doped optical fiber 38 is in an exited state,but the lights which can be amplified are the multiplexed lights at λ5to λ8 and the probe light only. The supervisory light signal at 1.48 μmpasses through the fiber after suffering from some loss.

Also, the pumping light source 39 is monitored by the optical detector57 ₃ for detecting a portion of the optical output from the pumpinglight source 39. At that time, a control unit 58 ₃ is configured tocontrol the devices so that the pumping light source supervisory unitsignal remains unchanged. The supervisory light signal at 1.48 μm havingpassed through the fiber is demultiplexed by the supervisory lightsignal multiplexer/demultiplexer 9 ₃, then being transmitted into asupervisory light signal path. The multiplexed lights, having passedthrough the optical multiplexer 37 and an optical isolator 60 ₃ insidethe optical amplifier 31, are multiplexed by an optical multiplexer 62 ₃with a pumping light from a semiconductor laser as a pumping lightsource 61 ₃ having the oscillation wavelength in proximity to 980 nm,thus being amplified by an erbium-doped optical fiber 63 ₃. Also, thepumping light source 61 ₃ is monitored by an optical detector 64 ₃ fordetecting a portion of the optical output from the pumping light source61 ₃. At that time, a control unit 65 ₃ is configured to control thedevices so that the pumping light source supervisory unit signal remainsunchanged.

The amplified signal lights, passing through an optical isolator 66 ₃,are introduced into a dispersion compensator 67 ₃. After being amplifiedby a second erbium-doped optical fiber 68 ₃, the signal lights passthrough an optical multiplexer 69 ₃, then being outputted from anoptical isolator 703. The second erbium-doped optical fiber 68 ₃ is inan exited state, since it is multiplexed with a pumping light from asemiconductor laser as a second pumping light source 71 ₃ having theoscillation wavelength in proximity to 980 nm. The multiplexed lightsfrom the optical isolator 70 ₃,, passing through an optical multiplexer35 and the supervisory light signal multiplexer/demultiplexer 9 ₂, areintroduced into the buffer light amplifying unit 10 ₂. The supervisorylight signal multiplexer/demultiplexer 9 ₂ multiplexes the supervisorylight signal at 1.48 μm wavelength and the signal lights.

The multiplexed lights introduced into the buffer light amplifying unit10 ₂ is introduced into the erbium-doped optical fiber as the rareearth-doped optical fiber 34, which is raised to be in an exited stateby a pumping light from the semiconductor laser as the pumping lightsource 33 having the oscillation wavelength in proximity to 980 nm. Thelights which can be amplified are the multiplexed lights at λ5 to λ8wavelengths and the probe light only. The supervisory light signal at1.48 μm passes through the fiber after suffering from some loss.

Also, the pumping light source 33 is monitored by the optical detector57 ₂ for detecting a portion of the optical output from the pumpinglight source 33. At that time, the control unit 58 ₂ is configured tocontrol the devices so that the pumping light source supervisory unitsignal remains unchanged. The amplified multiplexed lights and thesupervisory light signal at 1.48 μm are partially branched by theoptical coupler 28 the branching ratio of which is 5:95. A portion ofthe probe light, which has passed through a narrow bandwidth opticalfilter 46 allowing a probe light to pass through, is detected by anoptical detector 47.

The detected probe light supervisory unit signal is conveyed to theabove-mentioned control unit 59 ₃. The control unit 59 ₃ is configuredto control the pumping light source 71 ₃ so that the probe lightsupervisory unit signal remains unchanged. In this way, by controllingthe devices so that the probe light supervisory unit signal remainsunchanged, it becomes possible to control and maintain all the signallights at λ5 to λ8 wavelengths at fixed outputs. If any of the signallights at λ5 to λ8 wavelengths is cut off, or even if a signal lightother than the signal lights at λ5 to λ8 wavelengths is newly added, noinfluences are exerted on optical outputs of the signal lights at λ5 toλ8 wavelengths (for example, when a signal light at λ8 is cut off,signal lights at λ5 to λ7). This always makes it possible to perform afixed and stable control of the device.

The amplified multiplexed lights and the supervisory light signal havingpassed through the optical coupler 28 are transferred to thetransmission path 7 ₁, which is a single mode transmission fiber.

A part including the rare earth-doped optical fibers 34, 63 ₃, 68 ₃, 38,the pumping light sources 33, 61 ₃, 71 ₃, 39, and the opticalmultiplexers 32, 62 ₃, 40 may be replaced by a semiconductor opticalamplifier. In this case, it is advisable that an amplification ratio iscontrolled by a pumping electric current instead of the pumping lightsources 33, 61 ₃, 71 ₃, 39.

In the present configuration, a signal input power into the buffer lightamplifying unit 10 ₂ or 10 ₃ from the transmission path 7 ₁ or 7 ₂ fallsin a range of 5 dBm to 30 dBm, and a signal amplification gain in thebuffer light amplifying unit 10 ₂ or 10 ₃ is equal to an order of about10 dB. Since there is furnished no optical isolator within the bufferlight amplifying unit 10 ₂ or 10 ₃, an attention must be paid tooscillation phenomena of light. Accordingly, the signal amplificationgain in the buffer light amplifying unit 10 ₂ or 10 ₃ should be,preferably, 30 dB or less, or more preferably, 15 dB or less. Also, bymaking a positive gain the signal amplification gain in the buffer lightamplifying unit 10 ₂ or 10 ₃, a noise figure for a signal input from thetransmission path 7 ₁ or 7 ₂ in the intermediate repeater is obviouslyimproved as compared with the methods in the prior art, but morepreferably, the noise figure should be 5 dB or more.

Moreover, it is preferable that amplification gain distributions in thecore light amplifying unit 11 ₂ and the buffer light amplifying unit 10₂ or 10 ₃ should be calculated from a necessary output power into thetransmission path 7 ₁ or 7 ₂. For example, assuming that the outputpower into the transmission path 7 ₁ or 7 ₂ is equal to +11 dBm persignal wavelength, the total signal power (λ1 to λ4 and λp1, or λ5 to λ8and λp2) turns out to be +18 dBm, and consequently it is preferable thata power of the pumping light source 33 or 39 should be set to be about1.25 to 3.3 times as high as this power. When the power of the pumpinglight source is not enough, as illustrated in FIG. 5, the followingunits may be added, thereby providing a bi-directional pumping for theerbium-doped optical fibers: a new pumping light source 18-a, an opticaldetector 57-a for detecting the optical output thereof, a control unit58-a for keeping a detected supervisory unit signal unchanged, and anoptical multiplexer 17-a for introducing the pumping light. Besides, inany case, it is preferable that the pumping light sources 33, 39 whichcorrespond to a forward pumping for the multiplexed lights from thetransmission paths 7 ₁, 7 ₂, are furnished.

Based on the ability of the pumping light source 33 or 39 set above, itis possible to set an input power of the multiplexed lights, which areconveyed into the buffer light amplifying unit 10 ₂ or 10 ₃ from theoptical amplifier 31 or 36, at the value of the optical output from thebuffer light amplifying unit 10 ₂ or 10 ₃ (+11 dBm)−X dB. It ispreferable that a range of X should be 0 to 20. An adjustment of X makesit possible to set, at the above-mentioned more preferable value, asignal amplification gain for a signal input power which isreverse-directional, i.e. in a direction from the transmission path 7 ₁in the case of the buffer light amplifying unit 10 ₂, and in a directionfrom the transmission path 7 ₂ in the case of the buffer lightamplifying unit 10 ₃. Here, a 1480 nm semiconductor laser, which isadvantageous for a high power pumping, may be employed as the pumpinglight source 71 ₂ inside the optical amplifier 36, or as the pumpinglight source 71 ₃ inside the optical amplifier 31.

However, employment of the 980 nm semiconductor laser is desirable forthe pumping light source 33 inside the buffer light amplifying unit 10₂, the pumping light source 39 inside the buffer light amplifying unit10 ₃, the pumping light source 61 ₂ inside the optical amplifier 36, orthe pumping light source 61 ₃ inside the optical amplifier 31. As perthe above description, in much the same way the terminal stationrepeater 5 ₁ is applicable to the terminal station repeater 5 ₂, theintermediate repeater 6 ₁ is applicable to the intermediate repeaters 6₂, 6 ₃, etc.

With respect to a configuration of the terminal station repeater and anintermediate repeater according to the present invention, theconfiguration blocks included therein may be located outside the bufferlight amplifying unit. For example, FIG. 12 illustrates a configurationin which monitor light multiplexers/demultiplexers 9 ₂, 9 ₃, are locatedoutside a buffer light amplifying unit. Even in such a configuration, itis possible to obtain the effects given by the buffer light amplifyingunit according to the present invention. Regarding an insertion loss ofa signal light in the monitor light multiplexers/demultiplexers 9 ₂, 9₃, it should be set to be, more preferably, 1.9 dB or less, or even morepreferably, 0.4 dB.

Illustrated further in FIG. 13, as a partial derivative embodiment ofthe bi-directional optical transmission system illustrated in theabove-described FIG. 11, is a configuration embodiment of a buffer lightamplifying unit and core length amplifying unit in a single-directionaloptical transmission system. According to the present configuration, asignal light from a transmission path passes through a monitor lightmultiplexer/demultiplexer 9 ₂, then being introduced into the bufferlight amplifying unit. A monitor light demultiplexed by the monitorlight multiplexer/demultiplexer 9 ₂ is multiplexed with an infinitesimalan detectable-enough signal light which, being not completelydemultiplexed, is left behind. An optical multiplexer 88 extracts onlythe signal light from this, and a bandwidth passing light filter 46 andan optical detector 47 detects the signal light input. In the bufferlight amplifying unit 10 ₂, a demultiplexed signal light, after beingamplified by a rare earth-doped optical fiber 34, is introduced into thecore light amplifying unit 11 through an optical multiplexer 32. Therare earth-doped optical fiber 34 is the same as the rare earth-dopedoptical fiber in FIG. 11 in that it is pumped by a pumping light source33. Also, as is the case with FIG. 11, a portion of a signal lightamplified by the core light amplifying unit 11 is partially branched byan optical brancher 89. The signal light amplified by the core lightamplifying unit 11 is configured to be introduced again into thetransmission path through the monitor light multiplexer/demultiplexer 9₂.

In the buffer light amplifying unit in the present configuration, thereis no need of so much signal gain. This makes it possible to obtain aneffect of power of the pumping light source 33 even if the outputthereof is comparatively low. Accordingly, for example, the followingconfiguration is allowable. By regarding the pumping light source 33 asa pumping light source 61 ₃ or regarding the pumping light source 33 asa pumping light source 71 ₃, the pumping light source power isdistributed into the two light amplifying units. In that case, it isadvisable that a lower pumping light source power should be distributedinto the buffer light amplifying unit. The above-described configurationof the present invention is very effective in uni-directional opticaltransmission systems.

A simple calculation makes it possible to verify the effectiveness ofthe present configuration. For example, when the signal input is set tobe −27 dBm, the value of NF according to the conventional method turnsout to be 7 dB or more even if the insertion loss in the monitor lightmultiplexer/demultiplexer 9 ₂ is assumed to be 0.4 dB and the value ofNF in the rare earth-doped optical fiber is assumed to be 3.5 dB. On theother hand, the value of NF made possible by the configurationillustrated in FIG. 13 has been found to be 3.86 dB, assuming that theinsertion loss in the monitor light multiplexer/demultiplexer 9 ₂ is 0.4dB, a gain in the buffer light amplifying unit 10 ₂ is 13 dB, a gain ina previous-step rare earth-doped optical fiber 63 ₃ inside the corelight amplifying unit 11 is 15.5 dB, a loss in the dispersioncompensator 67 ₃ is 10 dB, and a gain in a next-step rare earth-dopedoptical fiber 68 ₃ inside the core light amplifying unit 11 is 18 dB.

Accordingly, the present configuration makes it possible to reduce atleast 3 dB of NF, as compared with the conventional configuration. Thus,converting from the signal S/N, it becomes possible to extend atransmission-possible distance by about 100 km or longer. Incidentally,in this trial calculation, a signal light output to a transmission fiber7 ₂ in FIG. 13 has turned out to be +6 to 8 dBm, which is extremelyclose to a value in an actual system.

Another embodiment of a one way transmission equipment of the presentinvention is described using FIGS. 14 and 15. FIGS. 14 and 15 are blockdiagrams that explain the embodiment of the one way transmissionequipment of the present invention. The difference between the twodifferent positions of the pumping light source that excites the dopedfiber of the buffer amplifier. That is, in the transmission equipment ofFIG. 14, it is backward pumping that is adverse with the transmissiondirection of the signal light. on the other hand, in the transmissionequipment of FIG. 15, it is forward pumping that is the same as thetransmission direction of the signal light. Because general and forwardexcitation is considered as a low noise, only FIG. 15 is explained here.But all the contents are also common to the embodiment of FIG. 14.

The example illustrated in FIG. 15 is the example of transmissionequipment that reduced three pumping light sources 33, 61 and 71 usedwith the one way transmission equipment of FIG. 13 to two pumping lightsources and planned economization. The process that the signal light isamplified is quite similar to the embodiment of FIG. 13, and descriptionis omitted. 120 MW pumping light is conducted to coupler 120 of which abranching ration is 2:8 that excite impurity doped fiber 34 from thepumping light source 33 in the structure of this example. The pumpinglight, from the port of which the branching ratio 2 of coupler 120,excites impurity doped fiber 34 of the buffer amplifier. The pumpinglight, from the port of which the branching ratio 8 of coupler 120,excites impurity doped fiber 63 ₃ of the core amplifier. And, in thisembodiment, 0.98 μm.

The gain of the buffer amplifier is acceptable at 10-16 dB, and thefiber length of impurity doped fiber 34 is also acceptable at 3-6 m.When the buffer amplifier is made high excitation, an optical isolatorbecomes necessary for an input step to the contrary, and it is contraryto the purposes of a present invention. And, the gain of impurity dopedfiber 63 ₃ of the core amplifier is 10-20 dB and fiber length 10-20 m.

Because in this embodiment, a fiber of which dispersion is large ispresupposed in 1.5 μm band as transmission fibers 7 ₁ and 7 ₂,dispersion compensator 67 ₃ is used. Therefore, to supply signal loss bydispersion compensator 67 ₃, the other impurity doped fiber 68 ₃ isinstalled in the core amplifier. It is clear that providing transmissionequipment for a transmission line using DSF with few dispersion in 1.5μm band, renders unnecessary the dispersion compensator 67 ₃, impuritydoped fiber 68 ₃ and pumping light source 71 ₃.

There is amplification equipment in the preceding phase of isolator 60 ₂in this example. As a result NF with the whole transmission equipmentcan be greatly improved. NF was 7.0 dB with the designed transmissionequipment in which a buffer amplifier was not installed. In comparisonwith this, by setting the buffer amplifier, NF greatly improved with 4.9dB or less. In additional, because the pumping light source of thebuffer amplifier and the core amplifier can be common, economicaltransmission equipment can be obtained.

The present invention, when applied to an optical transmission systemincluding terminal station repeaters and intermediate repeaters, makesit possible to provide an optical transmission device which is capableof performing a long haul transmission with a high reliability. Also,the present invention makes it possible to provide a long haul opticaltransmission system with high reliability.

In all embodiments described above, the relationship between the dopedfiber and the pumping light sources does not limit the structuresillustrated in the drawings. This is true even if the bidirectionalpumping, backward pumping, or forward pumping is used.

While the present invention has been described in detail and pictoriallyin the accompanying drawings, it is not limited to such details sincemany changes and modification recognizable to these of ordinary skill inthe art may be made to the invention without departing from the spiritand scope of the invention, and all such modifications as would beobvious to one skilled in the art are intended to be included within thescope of the following claims.

We claim:
 1. An optical signal receiver which suppresses a noise figure,comprising: a first optical amplifier which receives an attenuatedsignal light, amplifies the attenuated signal light to a firstpredetermined level and outputs the amplified attenuated signal light asa signal light to an optical component, said first optical amplifierbeing a low gain amplifier, wherein said optical component attenuatesthe signal light and outputs the attenuated signal light to a secondoptical amplifier, and wherein said second amplifier amplifies theattenuated signal light from said optical component at a secondpredetermined level and does not amplify the attenuated signal lightfrom said optical component when the attenuated signal light received atsaid first optical amplifier has an optical power level less than athird predetermined level.
 2. An optical amplifier for amplifying anoptical signal from a transmission line, said optical amplifiercomprising: a first doped fiber which amplifies the optical signal at again of less than 16 dB; a first pumping light source which excites saidfirst doped fiber; an optical component which provides an amplifiedoptical signal having a loss resulting from said first doped fiber; asecond doped fiber which amplifies the amplified optical signal fromsaid optical component at a gain more than the gain of said first dopedfiber; and a second pumping light source which excites said second dopedfiber and does not excite the second doped fiber when the optical signalfrom the transmission line has an optical power level less than apredetermined level.
 3. An optical amplifier for amplifying an opticalsignal from a transmission line, said optical amplifier comprising: afirst doped fiber which amplifies the optical signal and has a lengthless than 6 m; a first pumping light source which excites said firstdoped fiber; an optical component which provides an amplified opticalsignal having a loss resulting from said first doped fiber; a seconddoped fiber which amplifies the amplified optical signal from saidoptical component at a gain more than the gain of said first dopedfiber; and a second pumping light source which excites said second dopedfiber and does not excite the second doped fiber when the optical signalfrom the transmission line has an optical power level less than apredetermined level.
 4. An optical amplifier for amplifying an opticalsignal from a transmission line, said optical amplifier comprising: afirst doped fiber which amplifies the optical signal; an opticalcomponent which provides an amplified optical signal having a lossresulting from said first doped fiber; a second doped fiber whichamplifies the amplified optical signal from said optical component; apumping light source which generates a pumping light and does notgenerate a pumping light when the optical signal from the transmissionline has an optical power level less than a predetermined level; and anoptical separator which branches the pumping light to said first andsecond doped fibers, wherein said optical separator provides a largeamount of pumping light to said second doped fiber.